The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to a grid structure for providing cross mixing of the upward coolant flow.
The power output of a nuclear reactor is limited by the rate at which heat can be removed from the reactor core, and the rate of heat transfer determines the temperatures developed in a reactor core. The power output is also limited by the amount of structural material in the reactor core, as the structural material parasitically absorbes neutrons which could otherwise be used in the fission process.
One general structural form commonly used for providing a nuclear fuel inventory in nuclear reactors is that in which a plurality of elongated fuel elements or rods are arranged, within a prescribed volume, in a parallel array in an upstanding direction between the upper and lower reactor core plates. To provide integrity in the support relations, the fuel rods are divided into groups and the rods in each group are formed as a fuel assembly. Generally, in most reactors, a fluid coolant such as water, is directed upwardly through openings in the lower core support plate and along the fuel rods of the various fuel assemblies to receive the thermal energy therefrom.
Generally, heat is not generated uniformly in a reactor. The heat flux decreases axially and radially from a peak at the center of the reactor, or near the center if the reactor is not symmetrical in configuration, and this variation occurs even among the fuel rods within a single fuel assembly. The power variation results in variations in the enthalpy rise of the coolant among the different coolant flow channels throughout the reactor core. In addition, local pertubations in heat generation can occur because of inhomogeneities in the reactor structure. These variations impose special considerations in the design of the reactor cooling systems, including the need for establishing variations in coolant flow rate through the reactor to achieve uniform temperature rise in the coolant, avoiding local hot-spot conditions, and avoiding local thermal stresses and distortions in the structural members of the reactor. Along with these considerations, designers are always working toward minimizing the amount of structural materials in a fuel assembly for reducing the pressure drop to thereby increase the output of a reactor. Still further, due to the closeness of positioning of one fuel assembly among adjacent fuel assemblies in a reactor core, designers must take into consideration the structural integrity of each fuel assembly such that it can be installed and removed without interfering with an adjacent assembly.
In order to achieve a more uniform temperature rise in the coolant, avoid local hot-spot conditions and average the enthalpy rise to maximize power output, it has been found to be highly desirable to mix the coolant flow in a given fuel assembly, as well as in adjacent fuel assemblies. This was recognized by Tong, et al. in U.S. Pat. No. 3,395,077, which sets forth a grid structure, for a can-type fuel assembly, having openings therein through which the fuel elements extend and which is provided with planar mixing vanes for deflecting the coolant from one flow channel laterally across at least a portion of an adjacent coolant channel. The grid straps in Tong, et al. are all of the same height and the grid is axially supported by the walls of the vertically extending can structure. Among other shortcomings of the Tong, et al. device, the fuel rods have the tendency to come into damaging contact with the mixing vanes upon vibration of the rods and bending and bowing of the rods due to thermal induced stresses.
Based upon the teachings of the Tong, et al. patent, Andrews, et al. in U.S. Pat. No. 3,379,619, incorporated the mixing vanes directly on the positioning grids of a canless-type fuel assembly, such as the one shown in Creagan, et al. in U.S. Pat. No. 4,061,536. The positioning grids are mounted on the longitudinally extending control rod guide thimbles and support the fuel rods against lateral displacement and, to a given extent, frictionally against longitudinal movement. The use of the same device to both space the fuel rods and produce a lateral mixing flow places limitations on the mixing pattern that can be achieved. To increase the mixing or produce another flow pattern requires the introduction of yet another mixing support grid which has the disadvantage of adding further structural materials in resulting in an increase of the pressure drop. Yet another disadvantage of the Andrews, et al. device is that the corners of the grid interfere with and catch on the respective corners of the grids of adjacent fuel assemblies as a fuel assembly is installed into and removed from the reactor core. This problem is intensified after irradiation of a fuel assembly for a given period of time.